Normally, you can simply download any app that has been uninstalled from your device provided that it is still available in the store. If it is not, then the process will be a lot longer and more technical.

However, I'm not sure if you're actually referring to just Cortana (which can't be uninstalled, but can be disabled when you're in an unsupported region in your phone settings) or some third party app called "Hey Cortana".